Maybe post where you got the source code next time
Ok, so, you want to compile SimpleMsgPack.pas
Adding this to the top is required:
{$mode delphi}
{$MACRO ON}
{$define RTLVersion := 25}
{$define CompilerVersion := 18}
Note, the 25&18 are just any number to get it compiling.
FreePascal doesn't like the Byte to Boolean conversions, so add this type:
type
PBOOLEAN = ^ByteBool;
SimpleMsgPack should compile, but, I have no idea if it will work.